Old Answer
z-index
(note the hyphen!) is the property that matters. Make sure you set it greater than the dialogue, and make sure you set it on the correct element. Here's how we do it:
#ui-datepicker-div
{
z-index: 1000; /* must be > than popup editor (950) */
}
API Change - April 17, 2010
In the CSS file for the date picker, find the .ui-datepicker
item and change it:
.ui-datepicker { width: 17em; padding: .2em .2em 0; z-index: 9999 !important; }
Using z-index: 9999 !important; worked in Firefox 3.5.9 (Kubuntu).